Polymer actuator

ABSTRACT

A polymer actuator is provided. The polymer actuator includes a plurality of gel/electrode complexes arranged in an electrolytic solution, wherein the gel/electrode complex is composed of a polymeric hydrogel containing acidic or basic functional groups and electrodes placed in the polymeric hydrogel, such that it changes in volume upon application of a voltage across said electrodes. The polymer actuator expands and contracts in the linear direction without curved deformation. The polymer actuator is light in weight and is capable of control with a low voltage.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent Document No.P2002-357805 filed on Dec. 10, 2002, the disclosure of which is hereinincorporated by reference.

BACKGROUND

The present invention relates to a polymer actuator.

The usefulness of robots is attracting attention in various fieldsincluding nursing care service, dangerous work, and entertainment.Robots suitable for these uses are required to have articulations(movable parts) similar to those of animals that permit complexmovements.

A conventional actuator to drive these movable parts is a magneticrotary motor. This actuator, however, suffers the disadvantage of beingheavy because it is made of metal. Weight of actuators built intomovable parts add to loads. Heavy actuator needs large outputs, andpowerful actuators are large and heavy. Moreover, magnetic rotary motorsrequire speed reducers to control rotating speed and torque. Speedreducers deteriorate with time as gears therein wear out. Ultrasonicmotors producing a high torque at a low rotating speed do not need speedreducers; but they are also heavy because they are made of metal.

For this reason, there have recently been developed polymer actuators inwhich a light flexible polymeric material plays an important role. Theyinclude polymeric piezoelectric elements (which employ polyvinylidenefluoride), conducting polymer actuators (which employ electronconducting polymers), and gel actuators (which employ polymeric gel).

The gel actuator, particularly the one which employs a water-swellingpolymeric hydrogel, relies for its action on a polymeric hydrogel whichchanges in volume in response to temperature, ionic strength, and pH inits environment. The amount of change in volume is 30 to 50% and thechange in volume generates a force of 0.3 to 0.4 MPa. This performanceis comparable to that of skeletal muscles. The polymeric hydrogel,however, has some disadvantages. It cannot be heated or cooled rapidly.It needs an electrolytic solution to control ion strength and pH, whichhas to be circulated by a pump and stored in a reservoir. Consequently,it is not suitable for small, light systems.

There is another type of polymeric hydrogel, which is called apH-responsive polymeric hydrogel. This hydrogel is characterized in thatthe polymer molecules constituting it have acidic or basic functionalgroups, so that it changes in volume and swelling degree depending onthe pH of its surrounding aqueous solution. The one having acidicfunctional groups works in the following way. When it is in anelectrolytic aqueous solution with a high pH, the acidic groupsdissociate protons to become anions, thereby increasing inhydrophilicity and generating repulsive forces in or between negativelycharged molecules. This causes the gel to swell. Conversely, in anelectrolytic aqueous solution with a low pH, the acidic groups in thegel do not dissociate but form hydrogen bond between them. This causesthe gel to shrink.

By contrast, a pH-responsive polymeric hydrogel which have basic groupsworks in an opposite way. That is, in an electrolytic aqueous solutionwith a high pH, the basic groups in the gel protonize to become cations,thereby increasing in hydrophilicity and generating repulsive force inor between positively charged molecules. This causes the gel to swell.

Thus, when in use, the pH-responsive polymeric hydrogel is immersed inan electrolytic aqueous solution, and a voltage of about 1 to 3 V isapplied across electrodes placed therein. This voltage forms an ionconcentration gradient in the electrolytic aqueous solution and changesthe pH value in the neighborhood of the electrodes. This mechanism makesit possible to control the swelling and contraction of the pH-responsivepolymeric hydrogel only with a low voltage (e.g., 1 to 3 V) withoutrequiring heating and cooling units, pumps, and reservoirs.

The foregoing principle is put into practice as shown in FIG. 5. Thereis shown a container 10 holding an electrolytic aqueous solution 11. Thecontainer 10 is provided with two electrodes 12 a and 12 b. Between thetwo electrodes is placed a pH-responsive polymeric hydrogel 13. Uponapplication of a voltage across the electrodes 12 a and 12 b, the pH ofthe electrolytic aqueous solution 11 in the neighborhood of theelectrode 12 b (anode) increases and the gel 13 close to the electrode12 b swells. At the same time, the pH of the electrolytic aqueoussolution 11 in the neighborhood of the electrode 12 a (cathode)decreases and the gel 13 close to the electrode 12 a shrinks. As theresult, the gel 13 curves and deforms. The pH-responsive polymerichydrogel curves and deforms in the opposite direction if it is composedof polymer having basic groups.

The deformation that takes place as mentioned above may be used for anactuator. In fact, there is known an actuator which electrochemicallyproduces curved displacement from a pH-responsive polymeric hydrogelfilm held between electrodes connected to a voltage source. (SeeJapanese Patent Publication No. Hei-7-97912.) Incidentally, thisactuator produces a force of about 0.01 mPa due to curved displacementin the lengthwise direction.

Unfortunately, the actuator of curved displacement type is hardlyapplicable to robot articulations unlike the actuator capable ofextending and contracting in the linear direction like skeletal muscles.Moreover, the force produced from curved deformation is usually weak.

The gel can be made to expand (elongate) and contract without curving ifthe distance between electrodes is increased and the gel is broughtnearer to one electrode so that the gel is less affected by the otherelectrode. However, it is very difficult to fix the gel near oneelectrode while allowing the gel to expand and contract freely.

SUMMARY

The present invention was completed in order to address theabove-mentioned problems. The present invention provides a polymeractuator which is capable of expanding and contracting in the lineardirection without curving and which is also light in weight and capableof operation at a low voltage.

The present invention is concerned with a polymer actuator whichcomprises a plurality of gel/electrode complexes arranged in anelectrolytic solution, said gel/electrode complex being composed of apolymer gel containing acidic or basic functional groups and electrodesplaced in the polymer gel, such that it changes in volume uponapplication of a voltage across said electrodes.

Being constructed as mentioned above, the polymer actuator of thepresent invention obviates the necessity for heating and cooling units,pumps, and reservoirs unlike conventional ones, and it is light inweight and capable of control with a low voltage, such as 1 to 3 V.Moreover, it is capable of expansion and contraction in the lineardirection like skeletal muscles without curved displacement unlikeconventional ones.

The gel/electrode complex changes in volume to generate a force thatactuates robots' articulations (movable parts).

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic perspective view showing the structure of thepolymer actuator according to one embodiment of the present invention.

FIGS. 2A and 2B are schematic sectional views showing the polymeractuator according to one embodiment of the present invention. They showthe polymer actuator in its expanded (elongated) state and contractedstate, respectively.

FIGS. 3A and 3B are schematic perspective views showing gel/electrodecomplex constituting the polymer actuator according to one embodiment ofthe present invention. They show the gel/electrode complex in itsexpanded (elongated) state and contracted state, respectively.

FIGS. 4A and 4B are schematic perspective views showing gel/electrodecomplex constituting the polymer actuator according to anotherembodiment of the present invention. They show the gel/electrode complexin its expanded (elongated) state and contracted state, respectively.

FIG. 5 is a schematic sectional view showing a conventional polymeractuator.

DETAILED DESCRIPTION

According to an embodiment of the present invention, the polymer gelshould preferably be composed of a polymeric hydrogel, and theelectrolytic solution should preferably be an electrolytic aqueoussolution.

The polymer actuator according to an embodiment of the present inventionshould preferably be constructed of more than one unit of thegel/electrode complex, with the gel containing acidic functional groups,and more than one unit of the gel/electrode complex, with the gelcontaining basic functional groups.

FIG. 1 is a schematic perspective view showing the structure of thepolymer actuator 1 according to the present invention. FIGS. 2A and 2Bare schematic sectional views showing the polymer actuator 1 accordingto the present invention.

As shown in FIG. 1 and FIGS. 2A and 2B, the polymer actuator 1 accordingto the present invention should preferably be constructed of agel/electrode complex 4 a and a gel/electrode complex 4 b. Thegel/electrode complex 4 a is composed of a polymeric hydrogel 2 a havingacidic functional groups and an electrode 3 a placed in the polymerichydrogel 2 a, 1 occasionally referred to as an acidic gel/electrodecomplex hereinafter. The gel/electrode complex 4 b is composed of apolymeric hydrogel 2 b having basic functional groups and an electrode 3b placed in the polymeric hydrogel 2 b.

The gel/electrode complexes 4 a and 4 b are arranged in the container 5,which is filled with the electrolytic aqueous solution 6. The electrodes3 a and 3 b pass through the ends of the container 5. Incidentally, thegel/electrode complexes 4 a and 4 b are approximately parallel to eachother. The container 5 should preferably be capable of expanding andcontracting in response to the volume change of the gel/electrodecomplexes 4 a and 4 b.

The gel/electrode complexes 4 a and 4 b change in volume uponapplication of voltage across the electrodes 3 a and 3 b, said voltagechanging the pH value of the electrolytic aqueous solution in thevicinity of the gel/electrode complexes 4 a and 4 b.

The electrode 3 a of the acid gel/electrode complex 4 a may function asa cathode, and the electrode 3 b of the basic gel/electrode complex 4 bmay function as an anode. Application of a voltage (1 to 3 V) acrossthese electrodes changes the pH value as follows. The pH value of theelectrolytic aqueous solution 6 in the vicinity of the electrode 3 a(cathode) decreases, causing the acidic gel/electrode complex 4 a tocontract on account of hydrogen bonds forming between acidic groups(which remain undissociated). On the other hand, the pH value of theelectrolytic aqueous solution 6 in the vicinity of the electrode 3 b(anode) increases, causing the basic gel/electrode complex 4 b tocontract on account of hydrogen bonds forming between basic groups(which remain unprotonized). (See FIG. 2B)

When voltage is removed or when voltage polarity is reversed, the pHvalue of the electrolytic aqueous solution in the vicinity of the acidicgel/electrode complex 4 a increases. The pH increase causes the acidicgroups in the acidic gel/electrode complex 4 a to become anions, withthe acidic groups dissociating protons, thereby making the acidic gelmore hydrophilic and increasing intra- or intermolecular repulsive forcedue to negative charges. As the result, the acidic gel/electrode complex4 a expands. On the other hand, the pH value of the electrolytic aqueoussolution in the vicinity of the basic gel/electrode complex 4 bdecreases. The pH decrease causes the basic groups in the basicgel/electrode complex 4 b to become cations, with the basic groupsprotonized, thereby making the basic gel more hydrophilic and increasingintra- or intermolecular repulsive force due to positive charges. As theresult, the basic gel/electrode complex 4 b expands. (See FIG. 2A.)

The polymer actuator 1 according to the present invention has thegel/electrode complexes 4 a and 4 b arranged in the electrolytic aqueoussolution 6, so that the gel/electrode complexes 4 a and 4 b change involume upon application of a voltage across the electrodes 3 a and 3 bof the gel/electrode complexes 4 a and 4 b, as mentioned above.Therefore, it obviates the necessity for cooling and heating units,pumps, and reservoirs, and it is light in weight and is capable ofcontrol at a low voltage (say, 1 to 3 V).

In addition, the gel/electrode complexes 4 a and 4 b are constructed ofpolymeric hydrogels 2 a and 2 b having acidic or basic functionalgroups, and the electrodes 3 a and 3 b arranged in the polymerichydrogels 2 a and 2 b. Consequently, both of the gel/electrode complexes4 a and 4 b expand and contract in the same direction upon voltageapplication. The gel/electrode complexes 4 a and 4 b in a rodlike shapeas illustrated expand and contract in the lengthwise direction withoutcurved displacement unlike the conventional ones.

The gel/electrode complexes 4 a and 4 b change in volume in such a waythat the polymeric hydrogels 2 a and 2 b do not separate from theelectrodes 3 a and 3 b. Thus, the pH change that occurs in the vicinityof the electrode is efficiently transmitted to the gel/electrodecomplexes 4 a and 4 b. This leads to efficient expansion andcontraction. The volume change of the gel/electrode complexes 4 a and 4b generates a force large enough to actuate robot's articulations(movable parts).

The polymer actuator 1 according to the present invention has thegel/electrode complexes 4 a and 4 b, in which the electrodes 3 a and 3 bshould preferably be an electron conductor which follows the volumechange of the polymeric hydrogels 2 a and 2 b. For example, they shouldpreferably be a coiled metal wire, which undergoes elastic deformationto follow the volume change of the gel/electrode complexes 4 a and 4 b.The coiled metal wire should preferably be as thin as possible so thatit easily follows the volume change.

The electrodes 3 a and 3 b should preferably be formed from a materialwhich does not oxidize or reduce (for dissolution and passivation) uponvoltage application. Examples of such a material include gold, platinum,palladium, amorphous carbon, and graphite, with the last two beingpreferable because of light weight.

The electrolytic aqueous solution 6 may be an aqueous solutioncontaining any known water-soluble electrolyte dissolved therein. Thehigher the concentration of electrolyte, the higher the ion conductivityand the faster the pH change. However, the electrolytic aqueous solution6 with a high concentration absorbs water from the polymeric hydrogels 2a and 2 b due to difference in osmotic pressure. This prevents theexpansion of the gel/electrode complexes 4 a and 4 b. By contrast, theelectrolytic aqueous solution 6 with a low concentration does notprevent the expansion of the gel/electrode complexes 4 a and 4 b, but itmay be slow in response. Therefore, the concentration of electrolyteshould preferably be 0.01 to 0.5 mol/dm3.

The container 5 is one which holds the electrolytic aqueous solution 6.It also functions as a terminal which converts the displacement of thegel/electrode complexes 4 a and 4 b into a mechanical work. Thecontainer 5 may broadly vary in shape and material; it is required totightly seal the electrolytic aqueous solution 6 and to be flexibleenough to permit the displacement of the gel/electrode complexes 4 a and4 b. It should preferably be a baglike container made of film ofpolymeric material such as polyethylene, polypropylene, polyvinylchloride, polyvinylidene chloride, and fluorocarbon resin.

The terminals 7 electrically connected to the electrodes 3 a and 3 b ofthe gel/electrode complexes 4 a and 4 b project from the container 5without aggravating the sealability of the container 5. Since thecontainer 5 is flexible enough to follow the volume change of thegel/electrode complexes 4 a and 4 b, it changes in shape in response tothe displacement of the gel/electrode complexes 4 a and 4 b whose endsare fixed to it. Therefore, the ends 5′ of the container may be fixed toa mechanism (not shown) to be moved for mechanical work. The ends 5′ mayalso work as the electrode terminals mentioned above.

If the container 5 is not flexible, the ends 5′ may not be fixed to thecontainer 5 but may be slidable along the wall surface of the container5, so that the container 5 follows the volume change of thegel/electrode complexes 4 a and 4 b.

In the polymer actuator according to the present invention, the polymergel constituting the gel/electrode complexes may contain both acidicfunctional groups and basic functional groups. In this case, expansiontakes place as the basic groups cationize when the pH value of theelectrolytic solution is low in the vicinity of the gel/electrodecomplex. When the pH value is high, expansion also takes place as theacidic groups anionize. However, in the neutral region, both ionizedfunctional groups form ion complexes through ionic bond, which resultsin contraction.

This phenomenon also occurs in the case where the polymer gelconstituting the gel/electrode complexes is composed of a polymer havingacidic functional groups and a polymer having basic functional groups.

The gel in the gel/electrode complex expands in the neutral electrolyticsolution when the electrode potential is either noble or base; however,it contacts when the electrode potential is restored to an equilibriumpotential. This obviates the necessity for two kinds of gels for cathodeand anode. Thus, the gel/electrode complexes produce displacement in thelinear direction upon application of voltage across the electrodes ofthe gel/electrode complexes of the same structure.

The electrode for the gel/electrode complex is not limited to the coiledmetal wire illustrated above. It may be any electron conductor thatfollows the volume change of the polymer gel. An example is a metal meshshown in FIGS. 4A and 4B. The metal mesh will be able to follow thevolume change of the polymer gel through its elastic deformation as inthe case of metal wire mentioned above. The metal mesh should be formedfrom fine metal wires to ensure the net's flexibility.

The electrode constituting the gel/electrode complexes may be formedfrom an electrically conductive granular or fibrous material mixed withor dispersed in the polymer gel. Such an electrode effectively inducesthe pH change without interfering with the volume change of the polymergel.

Moreover, the electrode constituting the gel/electrode complexes may beformed from a coiled metal wire or a metal mesh and an electricallyconductive granular or fibrous material in combination with each other.The resulting electrode will induce the pH change more rapidlythroughout the gel/electrode complexes, thereby making the actuator torespond much faster.

The material constituting the electrodes should not oxidize or reduce(for dissolution and passivation) upon voltage application. Examples ofsuch a material include gold, platinum, palladium, amorphous carbon, andgraphite, with the last two being preferable because of light weight.

The polymer gel having acidic or basic functional groups whichconstitutes the gel/electrode complexes include polymers having acidicfunctional groups, such as carboxylic acid and sulfonic acid, andpolymer having basic functional groups, such as primary amine andsecondary amine.

Examples of the polymers having acidic functional groups includepolyacrylic acid, polymethacrylic acid, polyvinyl acetate, polymaleicacid, polyvinylsufonic acid, and polystyrenesulfonic acid.

Examples of the polymers having basic functional groups includepolyethyleneimine, polyallylamine, polyvinylpyridine, polylysine,polyvinylimidazole, poly(aminoethyl acrylate), poly(methylaminoethylacrylate), poly(dimethylaminoethyl acrylate), poly(ethylaminoethylacrylate), poly(ethylaminoethyl acrylate), poly(diethylaminoethylacrylate), poly(aminoethyl methacrylate), poly(methylaminoethylacrylate), poly(dimethyaminoethyl methacrylate), poly(ethylaminoethylmethacrylate), poly(ethylmethylaminoethyl methacrylate),poly(diethylaminoethyl methacrylate), poly(aminopropyl acrylate),poly(methylaminopropyl acrylate), poly(dimethylaminopropyl acrylate),poly-(ethylaminopropyl acrylate), poly(ethylmethylaminopropyl acrylate),poly(diethylaminepropyl acrylate), poly(aminopropyl methacrylate),poly(methylaminopropyl methacrylate), poly(dimethylaminopropylmethacrylate), poly(ethylaminopropyl methacrylate),poly(ethylmethylaminopropyl methacrylate), and poly(diethylaminopropylmethacrylate).

If necessary, these polymers may have intra- or intermolecularcrosslinking or may be mixed with other polymers. They may also be inthe form of copolymer composed of different monomers.

The gel/electrode complexes are not limited in their number so long asat least two units are used in combination. In the case where more thanone acidic gel/electrode complex and more than one basic gel/electrodecomplex are used in combination, it is necessary that the number of theformer be equal to the number of the latter because the former is givennegative voltage and the later is given positive voltage.

In the case where the polymer gel constituting the gel/electrodecomplexes contains a polymer having acidic functional groups and basicfunctional groups, or in the case where the polymer gel constituting thegel/electrode complexes contains a mixture of polymers each havingacidic functional groups and basic functional groups, the total numberof the gel/electrode complexes should preferably be even so that half ofthem are given negative voltage and another half of them are givenpositive electrode.

EXAMPLES

The invention will be described with reference to the following examplesin which the polymer actuator of the present invention was actuallyprepared and operated.

Example 1

In this example, the polymeric hydrogel for the gel/electrode complexesis prepared from an aqueous solution of monomer, crosslinking agent, andinitiator by radical polymerization.

The monomer for the polymer having acidic functional groups is acrylicacid. The crosslinking agent is N,N′-methylenebisacrylamide. Theinitiator is ammonium persulfate. The aqueous solutions (as the gelprecursor) is composed of the monomer, crosslinking agent, and initiatorin a molar ratio of 100:3:1.

The electrode is a coil (1 mm in diameter) of platinum wire (10 μm indiameter). It is placed in a glass tube, 1.5 mm in inside diameter and30 mm long, and then it is fixed so that the axis of the coil coincideswith the axis of the glass tube.

The glass tube is filled with the gel precursor solution. With its bothends closed by rubber stoppers, the filled glass tube is heated at 50°C. so that the gel precursor solution undergoes gelation. The resultinggel is discharged by applying pressure to one end of the glass tube.Thus there is obtained the acidic gel/electrode complex (in which thepolymer gel contains acidic functional groups).

Also, another gel precursor solution is prepared fromdimethylaminomethyl acrylate (as a monomer for the polymer having basicfunctional groups), N,N′-methylenebisacrylamide (as a crosslinkingagent), and ammonium persulfate (as an initiator). The gel precursorsolution is composed of the monomer, crosslinking agent, and initiatorin a molar ratio of 100:3:1. It is made into the basic gel/electrodecomplex (in which the polymer gel contains basic functional groups) inthe same way as mentioned above for the acidic gel/electrode complex.

The thus obtained acidic gel/electrode complex and basic gel/electrodeare immersed in 0.1 N aqueous solution of NaCl for 24 hours. Each ofthem is placed into a tubular polyethylene film, 6 mm in diameter and 50mm long. With one end heat-sealed, the tubular polyethylene film isfilled with a 0.1 N aqueous solution of NaCl, and the other end isheat-sealed. Incidentally, heat sealing is performed in such a way thatthe coil extending outward from the ends of the tubular polyethylenefilm is fixed in the polyethylene film. The projecting end of the coilserves as the electrode terminal. Thus there is obtained the polymeractuator as shown in FIG. 1 and FIGS. 2A and 2B.

The resulting polymer actuator, in which the acidic gel/electrodecomplex serves as a cathode and the basic gel/electrode complex servesas an anode, was tested by applying a voltage of 3 V across the anodeand cathode. It was found that polymer actuator decreased in lengthbetween the heat-sealed parts from 45 mm to 25 mm as the result ofcontraction of both of the gel/electrode complexes. The change in lengthtook place within 45 seconds. The force generated by the displacement inthe lengthwise direction was about 0.3 MPa.

Example 2

In this example, the polymeric hydrogel for the gel/electrode complexesis prepared from an aqueous solution of monomer, crosslinking agent, andinitiator by radical polymerization.

The monomer for the polymer having acidic functional groups is acrylicacid. The monomer for the polymer having basic functional groups isdimethylaminomethyl methacrylate. The crosslinking agent isN,N′-methylenebisarcylamide. The initiator is ammonium persulfate. Theaqueous solution (as the gel precursor) is composed of the acidicmonomer, basic monomer, crosslinking agent, and initiator in a molarratio of 50:50:3:1.

The electrode is a coil (1 mm in diameter) of platinum wire (10 □m indiameter). It is placed in a glass tube, 1.5 mm in inside diameter and30 mm long, and then it is fixed so that the axis of the coil coincideswith the axis of the glass tube.

The glass tube is filled with the gel precursor solution. With its bothends closed by rubber stoppers, the filled glass tube is heated at 50°C. so that the gel precursor solution undergoes gelation. The resultinggel is discharged by applying pressure to one end of the glass tube.Thus there is obtained the amphoteric gel/electrode complex (in whichthe polymer gel contains both acidic and basic functional groups).

Two units of the thus obtained amphoteric gel/electrode complex areimmersed in 0.1 N aqueous solution of NaCl for 24 hours. Each of them isplaced into a tubular polyethylene film, 6 mm in diameter and 50 mmlong. With one end heat-sealed, the tubular polyethylene film is filledwith a 0.1 N aqueous solution of NaCl, and the other end is heat-sealed.Incidentally, heat sealing is performed in such a way that the coilextending outward from the ends of the tubular polyethylene film isfixed in the polyethylene film. The projecting end of the coil serves asthe electrode terminal. Thus there is obtained the polymer actuator asshown in FIG. 1 and FIGS. 2A and 2B.

The resulting polymer actuator, in which one of the gel/electrodecomplexes serves as a cathode and the other of the gel/electrodecomplexes serves as an anode, was tested by applying a voltage of 3 Vacross the anode and cathode. It was found that polymer actuatorincreased in length between the heat-sealed parts from 30 mm to 45 mm asthe result of expansion of both of the gel/electrode complexes. Thechange in length took place within 44 seconds. The force generated bythe displacement in the lengthwise direction was about 0.3 MPa.

Example 3

In this example, the polymeric hydrogel for the gel/electrode complexesis prepared from an aqueous solution of monomer, crosslinking agent, andinitiator by radical polymerization.

The monomer for the polymer having basic functional groups isdimethylaminomethyl methacrylate. The polymer having acidic functionalgroups is polyacrylic acid. The crosslinking agent isN,N′-methylenebisarcylamide. The initiator is ammonium persulfate. Theaqueous solution (as the gel precursor) is composed of the basicmonomer, acidic polymer, crosslinking agent, and initiator in a molarratio of 50:50:3:1. The amount of polyacrylic acid is measured in termsof mol.

The electrode is a coil (1 mm in diameter) of platinum wire (10 □m indiameter). It is placed in a glass tube, 1.5 mm in inside diameter and30 mm long, and then it is fixed so that the axis of the coil coincideswith the axis of the glass tube.

The glass tube is filled with the gel precursor solution. With its bothends closed by rubber stoppers, the filled glass tube is heated at 50°C. so that the gel precursor solution undergoes gelation. The resultinggel is discharged by applying pressure to one end of the glass tube.Thus there is obtained the acidic-basic gel/electrode complex in whichthe polymer gel is a mixture of polymers each containing acidicfunctional groups and basic functional groups.

Two units of the thus obtained acidic-basic gel/electrode complex areimmersed in 0.1 N aqueous solution of NaCl for 24 hours. Each of them isplaced into a tubular polyethylene film, 6 mm in diameter and 50 mmlong. With one end heat-sealed, the tubular polyethylene film is filledwith a 0.1 N aqueous solution of NaCl, and the other end is heat-sealed.Incidentally, heat sealing is performed in such a way that the coilextending outward from the ends of the tubular polyethylene film isfixed in the polyethylene film. The projecting end of the coil serves asthe electrode terminal. Thus there is obtained the polymer actuator asshown in FIG. 1 and FIGS. 2A and 2B.

The resulting polymer actuator, in which one of the gel/electrodecomplexes serves as a cathode and the other of the gel/electrodecomplexes serves as an anode, was tested by applying a voltage of 3 Vacross the anode and cathode. It was found that polymer actuatorincreased in length between the heat-sealed parts from 30 mm to 45 mm asthe result of expansion of both of the gel/electrode complexes. Thechange in length took place within 44 seconds. The force generated bythe displacement in the lengthwise direction was about 0.3 MPa.

Example 4

The same procedure as in Example 1 was repeated to prepare the polymeractuator except that the gel precursor solution was incorporated withcarbon powder (crushed carbon fiber) corresponding to 5 wt % of themonomer weight.

The resulting polymer actuator was tested by applying a voltage of 3 Vto the acid gel/electrode complex (as the cathode) and the basicgel/electrode complex (as the anode). It was found that polymer actuatordecreased in length between the heat-sealed parts from 45 mm to 31 mm asthe result of expansion of both of the gel/electrode complexes. Thechange in length took place within 21 seconds. The force generated bythe displacement in the lengthwise direction was about 0.3 MPa.

Exploitation in Industry

The polymer actuator according to the present invention includes aplurality of gel/electrode complexes arranged in an electrolyticsolution, said gel/electrode complex being composed of a polymer gelcontaining acidic or basic functional groups and electrodes placed inthe polymer gel, such that it changes in volume upon application of avoltage across said electrodes. Therefore, the polymer actuator obviatesthe necessity for heating and cooling units, pumps, and reservoirsunlike conventional ones, and it is light in weight and capable ofcontrol with a low voltage, say 1 to 3 V.

Moreover, it is capable of expansion and contraction in the lineardirection like skeletal muscles without curved displacement unlikeconventional ones. The gel/electrode complex changes in volume togenerate a force that actuates robots' articulations (movable parts).

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1-13. (canceled)
 14. A polymer actuator comprising a plurality ofgel/electrode complexes arranged in an electrolytic solution, saidgel/electrode complex being composed of a polymer gel containing acidicor basic functional groups and electrodes placed in the polymer gel,such that the polymer actuator changes in volume upon application of avoltage across said electrodes.
 15. The polymer actuator as defined inclaim 14, wherein the electrolytic solution changes in pH value inproximity of the gel/electrode complexes upon voltage application acrossthe electrodes, and the gel/electrode complexes change in volume inresponse to the pH change.
 16. The polymer actuator as defined in claim14, further comprising more than one unit of the gel/electrode complexcomposed of a polymer gel having acidic functional groups and more thanone unit of the gel/electrode complex composed of a polymer gel havingbasic functional groups.
 17. The polymer actuator as defined in claim14, wherein the polymer gel constituting the gel/electrode complexcontains a polymer having acidic functional groups and basic functionalgroups.
 18. The polymer actuator as defined in claim 14, wherein thepolymer gel constituting the gel/electrode complex contains a mixture ofpolymers each having acidic functional groups and basic functionalgroups.
 19. The polymer actuator as defined in claim 14, wherein thegel/electrode complexes are arranged parallel to each other.
 20. Thepolymer actuator as defined in claim 14, wherein the gel/electrodecomplexes are arranged in a container which is filled with saidelectrolytic solution and said container has electrodes projecting fromits both ends.
 21. The polymer actuator as defined in claim 20, whereinthe container is flexible so as to follow the volume change of thegel/electrode complex.
 22. The polymer actuator as defined in claim 14,wherein the polymer gel is a polymeric hydrogel and the electrolyticsolution is an electrolytic aqueous solution.
 23. The polymer actuatoras defined in claim 14, wherein the electrode constituting thegel/electrode complex is a coiled metal wire or a metal mesh.
 24. Thepolymer actuator as defined in claim 14, wherein the electrodeconstituting the gel/electrode complex is any one of an electricallyconductive granular, and a fibrous substance mixed with or dispersed inthe polymer gel.
 25. The polymer actuator as defined in claim 14,wherein the electrode constituting the gel/electrode complex is composedof a coiled metal wire or a metal mesh and an electrically conductivegranular or fibrous substance.
 26. The polymer actuator as defined inclaim 14, wherein the electrode is made of at least one species selectedfrom the group consisting of gold, platinum, palladium, amorphouscarbon, and graphite.